Wireless communications systems can use transmit beamforming to improve the level of the signal seen at a desired receiver, as well as reduce the levels of interference seen at other receivers. The interference reduction capability of beamforming can be particularly advantageous in cellular wireless systems, where high levels of interference can severely reduce the capacity of such systems.
Beamforming generally refers to techniques used in wireless communications systems such as radio frequency, optical frequency or acoustic frequency systems wherein signals transmitted or received by multiple transmit or receive sensors are combined in such as way as to improve their overall gain, or carrier to interference ratio. Beamforming requires at least two transmit or receive sensors.
Beamforming has typically been used in cellular wireless communications systems to improve the range over which a mobile device can communicate with the base station. An additional possibility with beamforming is the ability to reduce interference by choosing phases and signal amplitudes that can cause the signals received at or transmitted by a different mobile station to cancel.
Beamforming typically employs multiple antennas at the base station and uses signal processing techniques to ensure that the phases of the signals are aligned with each other by the time that they reach the mobile device. In systems that use Time Division Duplexing, (TDD) where the same set of frequencies are used for both downlink (base station to mobile station) and uplink (mobile station to base station) transmissions, the base station can take advantage of the channel reciprocity to adjust the amplitudes and phases of the transmissions at each antenna. For Frequency Division Duplexing (FDD) systems, where different frequencies are used for the downlink and uplink transmissions, feedback from the mobile station to the base station about the amplitudes and phases of the signals received at the mobile station is generally required.
Cellular wireless beamforming systems typically use from two to eight antennas. Since the cost of supporting beamforming in a base station product increases as the number of antennas increase, systems with more than eight antennas have generally been regarded as being cost prohibitive.
FIG. 1 illustrates a perspective view of a wireless beamforming system 100 that uses two transmit antennas at a base station to communicate with a mobile station in accordance with an embodiment of the invention. Signal processing algorithms at the base station 102 choose the appropriate phases and amplitudes of the signals 104 and 106 at each of the base station transmit antennas to ensure that the combined signal received at the mobile station 108 has sufficient power to operate correctly.
FIGS. 2-5 show some examples of how the phase and amplitude differences between the two signals 104 and 106 arriving at a mobile device (e.g., 108), or a customer premise equipment (CPE), can impact the combined signal that the receiver sees. A beamforming system (e.g., 100) can control the relative amplitudes and phases at the transmitter so that the combined signal seen at the receiver (e.g., 108) can have increased amplitude, or can have reduced amplitude.
FIGS. 2A and 2B illustrate plots 202 and 204, respectively, where two sinusoidal signals of equal power with phase differences of 0° and 180° are being received at a mobile device (e.g., 108) in accordance with an embodiment of the present invention. In the first plot 202, the two signals 104 and 106 are perfectly aligned with each other in phase. The combined signal (marked with triangles) has twice the amplitude of the individual signals. In the second plot 204, the two signals are 180° out of phase with each other. In this case, the signals cancel each other out perfectly, resulting in a combined signal that has zero amplitude. The receiver (e.g., 108) in this case does not detect any signal due to the perfect cancellation of the signals through destructive interference.
FIGS. 3A and 3B illustrate plots 302 and 304, respectively, where two sinusoidal signals of unequal power with phase differences of 0° and 180° are being received at a mobile device (e.g., 108) in accordance with an embodiment of the present invention. In this example, the two signals are not equal in power but rather the first signal 104 is 3 dB stronger than the second signal 106. The first plot 302 depicts the two signals 104 and 106 perfectly aligned with each other in phase, resulting in a much stronger received combined signal. The second plot 304 shows the scenario where the two signals 104 and 106 are 180° out of phase with each other. In this scenario, the signals do not completely cancel each other out, but the combined signal at the receiver is still attenuated significantly when compared with the case of the two separate signals being aligned perfectly with each other (e.g., in plots 202 and 302).
It is not necessary that the signals arriving at the receiver be aligned exactly in phase in order for a combining gain in signal strength to be achieved. Likewise, it is not necessary that the signals be exactly 180° out of phase with each other to realize a signal cancellation. Thus, FIGS. 4A and 4B illustrate plots 402 and 404, respectively, where two sinusoidal signals of unequal power with phase differences of 45° and 160° are being received at a mobile device (e.g., 108) in accordance with an embodiment of the present invention
FIGS. 4A and 4B show the same two signals (e.g., signals 104 and 106) with the same 3 dB power difference as in FIGS. 3A and 3B, but this time the phase differences at the receiver (e.g., 108) are 45° and 160°. In the scenario where the signals are received with a 45° phase difference (i.e., in plot 402), the combined signal at the receiver still shows significant gain and is not much reduced when compared to the scenario where the signals are received with a 0° phase difference shown in plot 302 of FIG. 3A. Similarly, when the original signals are received 160° out of phase with each other (as depicted in plot 404), there is still a significant reduction in the level of the combined signal when compared to the scenario where the signals are received with a 180° phase difference shown in plot 304 of FIG. 3B.
FIG. 5 illustrates a plot 500 of the power gain of the combined signals, also known as the beamforming gain, versus the phase difference of two signals (e.g., signals 104 and 106) at a receiver (e.g., 108) in accordance with an embodiment of the present invention. Note that the plot 500 assumes that both signals are received with equal amplitude, similar to the signals depicted in FIGS. 2A and 2B. The beamforming gain is relative to a signal sent at a nominal level of 0 dB from one of the transmit antennas. The largest gain (6 dB) is seen when the two signals are perfectly aligned in phase (e.g., as in plot 202), while the lowest gain (in this case negative ∞ when expressed in dB) is seen when the signals have a phase difference of 180° (e.g., as in plot 204).
Modern wireless communication networks include many different network topologies comprising heterogeneous mixtures of macrocell, microcell, picocell, and femtocell resources. At the highest level of wireless coverage, a macrocell provides cellular service for a relatively large physical area, often in areas where network traffic densities are low. In more dense traffic areas, a macrocell may act as an overarching service provider, primarily responsible for providing continuity for service area gaps between smaller network cells. In areas of increased traffic density, microcells are often utilized to add network capacity and to improve signal quality for smaller physical areas where increased bandwidth is required. Numerous picocells and femtocells generally add to network capacity for even smaller physical areas in highly populated metropolitan and residential regions of a larger data communications network.
This mixture of larger and smaller cells can reduce periods of network congestion created by traditional network architecture which previously bottlenecked a majority of regional subscriber communications through a small number of larger network cells (e.g., macrocells or microcells). This congestion reducing technique can improve a service provider network's Quality of Service (QOS) as well as network service subscribers' collective Quality of Experience (QOE) within a particular portion of a data communications network. Negative effects associated with poor QOS and poor QOE (e.g., conditions largely caused by congestion and/or interference), which can be mitigated by adding a substantial number of short-range wireless base station devices to network infrastructure, may include: queuing delay, data loss, as well as blocking of new and existing network connections for certain network subscribers.
As the number of overlapping cells in a network increases (i.e., the number of macrocells, microcells, picocells, and femtocells in a network), it becomes increasingly important to manage the airlink resources shared by the components in a network. By way of example, resources such as frequency channels, timeslots, and spreading codes need to be managed for each cell in a network, and poor management can result in increased interference and a decrease in overall network efficiency.
Prior art systems have attempted to use beamforming techniques to manage a transmission from a base station to an intended mobile device to increase the signal strength similar to the techniques described in plots 202, 302, and 402. Some prior art systems have attempted to reduce undesired interference levels using beamforming signal cancellation techniques similar to the techniques described in plots 204, 304, and 404. However, these systems require relatively complex signal processing algorithms and communications between base stations to achieve the interference reductions. Thus, it would be desirable to schedule transmissions in a wireless network such that the signals received by a mobile device from a serving base station combine in a constructive manner at the mobile receiver, while signals arriving at a mobile base station from non-serving base stations combine in a destructive manner at the mobile receiver. Additionally, it would be desirable for the scheduling to be minimally resource-intensive so that complex scheduling can be easily performed.